There are many types of mobile nodes that need to communicate with a fixed-station communications system. One example of a mobile node is a cellular telephone. Typically, a cellular telephone is in constant communications with an access point. An access point is one of one or more base stations that enable a mobile node to interact with a communications system. Another example of a mobile node is a wireless networking node. A wireless networking node can include, but is not necessarily limited to an 802.11a networking node. These are but examples of the types of mobile nodes that typically need to interact with a communications system through an access point.
In a typical cellular communications system, a mobile node is limited in the amount of power that is able to radiate. By limiting the radiated power of a mobile node, it is fairly easy to ascertain when a particular mobile node is physically within a particular cell in the cellular system. As the mobile node migrates from one cell to the next, a base-station in one cell loses contact with the mobile node as a base-station in an adjacent cell establishes contact with the mobile node. As simplistic as this model may seem, it is quite effective and it is enabled by limiting the radiated power of the mobile node. If the radiated power of the mobile node were not limited in this manner, the base stations in two adjacent nodes could actually be confused with respect to which of those base stations is responsible for maintaining communications with the mobile node.
It should be appreciated that the simplistic model just described is not really how modern cellular communications systems work. Although the radiated power of a mobile node is limited, it is typically limited in order to prevent communications with a corresponding cell in a cellular matrix. A corresponding cell in a cellular matrix is typically thought of as a cell that has an identical set of adjacent cell types. For example, a mobile node is typically required to communicate with an adjacent cell before the adjacent cell acquires cognizance over the mobile node. Accordingly, the power limitation of a mobile node must not be so grave that this adjacent cell communication is precluded. A cell that is just beyond the most adjacent cell is typically considered to be a corresponding cell. It is the unwanted communications from the mobile node to this corresponding cell that is typically precluded by limiting the amount of power the mobile node is allowed to radiate.
It can be appreciated that the power that a particular mobile node is allowed to radiate must be governed by the topology of a cellular matrix and by other factors, e.g. a power limitation established by a regulatory agency. In a rural area, for example, a cellular matrix may include a cell that has a much greater spatial area than normal. It can be appreciated that as the spatial size of a cell decreases, the amount of power a mobile node can radiate will be accordingly limited. As such, a larger cell (e.g. one situated in a rural area) will allow a mobile node to radiate more power. To accommodate these variations, a base station serving a cell will broadcast a maximum power indicator that a mobile node uses to determine how much power the mobile node can radiate within the cell. This limitation is then honored by the mobile node. If the mobile node were to transmit at a much greater power level, the mobile node could interfere with communications in other cells in the system.
As prevalent as cellular communications systems are, they typically have a limited coverage area. For example, a cellular telephone system is generally intended to serve mobile nodes that are on the ground. An airborne mobile node is not generally supported by a cellular telephone system. A marine user is also generally not supported by a land-based cellular telephone system. Although airborne and marine mobile nodes can be within the range of a base station serving a cell in a cellular communications system, the fact that these mobile nodes lie outside of an intended coverage region results in some interesting problems.
The first problem is that of determining the amount of power that the mobile node is allowed to radiate. The fact that the mobile node can receive a signal from a base station only means that a satisfactory downlink can be achieved. The mobile node could use a power limitation indicator received from the base station, but if the mobile node were to limit its radiated power to this level it may not be able to establish an uplink to the base station if it is too far away from cell. Arbitrarily boosting its radiated output power could be even worse. A mobile node operating outside of the intended coverage region could interfere with other mobile nodes either situated in the cell or other cells adjacent thereto. In this case, the mobile node could actually introduce a radiated power density into the cell that exceeds limitations established by a regulatory agency.
Although cellular communications systems are common place, the problems of interacting with such systems from outside of an intended coverage area are common with other communications systems that only have a single base station. For example, an interaction with a single base station communications system can be just as problematic when the mobile node travels beyond the coverage area. Consider one example where a repeater is situated atop a mountain. So long as the mobile node limits its output power in accordance with established limitations, effective communications can be achieved. However, arbitrarily increasing output power to achieve an uplink can result in regulatory non-compliance and interference with other users.